For various medical reasons, patients are often put on mechanical or electronic ventilators which provide air, oxygen, or a mixture of air and oxygen to the patient. The gas mixture concentration is usually expressed as a percentage of oxygen. In surgery or for pain control, sometimes a mixture of oxygen and nitrous oxide is provided to the patient. Sometimes, gases other than nitrous oxide will be used, such as anesthetics. If an anesthetic or nitrous oxide is provided, it is important to accurately know the concentration of the gas provided to safeguard the health of the patient.
Thus, for example, U.S. Pat. No. 3,351,057 to Goodyear discloses an anesthesia apparatus which supplies oxygen, anesthesia or a combination of both, with a toggle mechanism which shuts off one gas supply and connects the device to ambient air in the event the oxygen supply is inadvertently shut off. The mechanism described, however, is complex and it requires the patient to draw the ambient air into the system.
Another patent, U.S. Pat. No. 3,896,837 to Rohling, discloses a gas mixing apparatus for respirators and medical devices with a gas bypass arranged such that when the pressure in a mixed gas delivery line drops below a predetermined pressure, a bypass valve opens to permit direct passage of an inlet gas, or to permit passage of a third gas from a gas tank. In Rohling, however, there is no regulation control on the added gas which can lead to an unknown mixture of gasses being delivered to the patient.
Continuous flow gas metering devices are known in the art For example, U.S. Pat. No. 4,266,573 to Braatz discloses an anesthesia machine for providing a mixture of oxygen and a second gas, such as nitrous oxide, in selected proportions. Each of the two gasses pass through separate needle valve metering assemblies which are interconnected so that when a predetermined ratio of nitrous oxide and oxygen is achieved, the percentage of oxygen may not be further diminished, thereby safeguarding the health of the patient. However, the needle valve assemblies are complex, and pressure variations can affect the relative flow rates.
Another gas metering device uses pulsed solenoid control valves, as described in U.S. Pat. No. 4,576,159 to Hahn. However, the pulsed valve actuation causes pressure surges which are so severe that specially designed mixing chambers must be used which dampen the pressure fluctuations of the mixed gases. The pressure fluctuation and pulsed gas disbursement can also lead to inaccuracies in the relative gas concentrations in the mixed gas.